Lightning Over-Voltages in Underground Cables

Lightning can impress a potential on the sheath of a buried cable either by direct stroke or by induction. This fact is by itself not sufficient to explain the occurrence of voltages between the sheath and the cores, since the cores are completely enclosed by the sheath and initially take up the potential of the latter by induction. Voltages between the sheath and the cores can occur only through the formation of travelling waves; it is shown that waves of at least two different velocities must occur. Soil conductivity and wave front are important factors. Various remedies are suggested.